1. Field of the Disclosure
This disclosure generally relates to an image system and, more particularly, to a BJT pixel circuit capable of cancelling ambient light influence, an image system including the same and an operating method thereof.
2. Description of the Related Art
Referring to FIG. 1, it is a schematic diagram of a conventional bipolar junction transistor (BJT) pixel circuit 100. The BJT pixel circuit 100 includes a photodiode 11, a BJT transistor 13, a pixel capacitor Cp, a power source VDD, a plurality of switching elements SWres, SWshut, SW0, SW0d and two source followers SF and SFd.
FIG. 2 is an operational timing diagram of an image sensor including the BJT pixel 100 of FIG. 1, and herein the sensor array having three pixel circuits P1, P2 and P3 is taken as an example for illustration purpose.
The BJT pixel circuit 100 may operate in two phases, including a reset phase and a shutter phase, by controlling the switching elements, wherein in the reset phase the switching elements SWres are conducted, while in the shutter phase the switching element SWshut is conducted. The reset phase is used to release charges left in the photodiode 11 and to charge the pixel capacitor Cp to an initial voltage Vint. The shutter phase is to allow the BJT pixel circuit 100 to discharge the initial voltage Vint and the discharging rate is a function of light intensity. FIG. 1 shows a state of the shutter phase which operates corresponding to a system light source 20. The system light source 20 is activated corresponding to a first frame A and deactivated corresponding to a second frame B (referring to FIG. 2), wherein the second frame B is sometimes referred to an ambient image frame.
In the first frame A, a period between the system light source 20 being activated (e.g. time t1) and the switching element SWshut being conducted (e.g. time t2) is referred to a pre-flash time, which is arranged to allow the BJT transistor 13 to achieve a stable output. After the switching element SWshut is conducted, charges held in the pixel capacitor Cp discharges via the BJT transistor 13 in an exposure time (e.g. time t2 to t3) to lower the initial voltage Vint, wherein a decrement of the initial voltage Vint corresponds to the light intensity received by the photodiode 11.
Then, the pixel circuit 100 outputs, via the switching element SW0, an output signal OUT. In addition, the pixel circuit 100 further outputs a reference signal OUTd via the switching element SW0d, wherein this reference signal OUTd is to cancel out switch injection and IR drop. F or example, in the first frame A, the pixel circuit 100 outputs a pair of output signal OUTon and reference signal OUTdon corresponding to each of the pixel circuits P1 to P3, i.e. three pairs of signals. Similarly, in the second frame B, the pixel circuit 100 outputs another pair of output signal OUToff and reference signal OUTdoff corresponding to each of the pixel circuits P1 to 93, i.e. another three pairs of signals.
The output signal OUT and the reference signal OUTd are then inputted into a transfer amplifier 17 for differential operation. Differential signals between the output signal OUT and the reference signal OUTd (e.g. OUTdon−OUTon and OUTdoff−OUToff) are converted into digital signals by an analog to digital converter (ADC) 18 and the digital signals are post-processed by a processor 19, wherein a sampling number of the ADC converter 18 is positively correlated with a size of the sensor array, e.g. three times per frame as shown in FIG. 2. In the conventional pixel circuit 100, in order to cancel the influence of ambient light, the processor 19 performs the subtraction operation on the differential signals at the digital back end, i.e. sum of ADC value_on−sum of ADC value_off.
However, the light intensity received by the photodiode 11 includes both system light and ambient light. When the ambient light is very strong (e.g. under sun light), the difference between the differential signal (OUTdon−OUTon) and the differential signal (OUTdoff−OUToff) becomes very small, even smaller than the resolution of the ADC 18, such that the processor 19 is unable to distinguish between the activation and deactivation of the system light source 20. Accordingly, when the pixel circuit 100 is applied to a proximity sensor, it is not possible to detect an approaching object under strong ambient light.